Generally, in work vehicles such as a wheel loader, an engine serves as a driving source for vehicle running and a driving source for a working unit. That is, the output of the engine that is transmitted to driving wheels via a torque converter allows a vehicle to run. In addition, the engine drives various types of hydraulic pumps including a hydraulic pump for the working unit, and these hydraulic pumps drive various types of devices such as the working unit via hydraulic actuators. Specifically, when the engine drives a steering hydraulic pump, pressure oil that is discharged from the steering hydraulic pump is provided to a steering hydraulic cylinder so that a steering mechanism can be driven, for example. Also, when the engine drives a loader hydraulic pump, pressure oil that is discharged from the loader hydraulic pump is provided to a loading hydraulic cylinder so that a loading device can be driven.
Thus, in wheel loaders, the output of one engine is used both for the vehicle running and the driving of various devices such as the working unit. Accordingly, the magnitude of engine output that is used for the vehicle running will vary according to the magnitude of a load that is applied to the working unit and the like.
For example, in the case where the rotational speed of the engine is in a low idle rotational speed range (idling state), as compared with the case where the rotational speed of the engine is in a high rotational speed range, rise of engine torque slowly responds to rise of a hydraulic load. For this reason, if an operator operates the loading device to raise the loading device carrying a load while operating a steering wheel which in turn applies a sharply-increased hydraulic load, the torque rise of the engine cannot respond to the sharply-increased hydraulic load, and the engine may stall.
Also, in the case where the loading device and the steering mechanism are activated during vehicle running, since engine output is consumed for hydraulic loads for the working unit and the steering mechanism, output that can be used for the vehicle running load is limited to the rest of engine output. For this reason, the response to accelerator operation by an operator gets worse, for example, sufficient pulling force may not be provided, or it may take time to raise the vehicle speed.
For example, Japanese Patent Laid-Open Publication No. 2004-108155 (published on Apr. 8, 2004) discloses a pump torque control device of a hydraulic construction machine that, during speed sensing control in which the maximum absorbing torque of a hydraulic pump is controlled based on the deviation between a target rotational speed and an actual rotational speed, if an engine has sufficient output, the output of the engine can be effectively utilized.